1. Field of the Invention
The present invention relates to a DC-DC converter having a synchronous rectifier and is suitably incorporated in, for example, a switching power supply unit.
2. Description of the Related Art
As well known in the art, a DC-DC converter incorporated in, for example, a switching power supply unit converts a DC input voltage Vin into an alternating current by a switching operation of a switching element (e.g., a MOS-FET) and rectifies and smooths the AC voltage by means of a rectifier/smoothing circuit, thereby delivering a D.C. voltage Vout to a load. This type of DC-DC converter permits the output voltage Vout to be variably controlled through a control of the switching operation performed by the switching element. In other words, the ratio of the output voltage Vout to the input voltage Vin (referred to as xe2x80x9coutput/input ratioxe2x80x9d) is determined by the switching operation of the switching element. For this reason, it is a common practice to control the switching operation of the switching element so as to stably control the output voltage Vout to a predetermined set voltage. In recent years, the use of a synchronous rectifier as the above-mentioned rectifying/smoothing circuit is becoming popular, in order to reduce a conduction loss.
This known arrangement has a risk that a voltage exceeding the output voltage Vout supplied by the DC-DC converter to the load (referred to as an overshoot voltage) is imposed on the output of the DC-DC converter due to, for example, a drastic rise of the input voltage Vin or a reduction in the current flowing through the load.
Application of such an overshoot voltage causes a smoothing capacitor of the rectifying/smoothing circuit to be charged by charges corresponding to the overshoot voltage. When the output voltage of the DC-DC converter settles again to the steady voltage due to extinction of the overshoot voltage, the smoothing capacitor commences discharging. In the meantime, the switching operation of the switching element has been controlled so as to reduce the output voltage Vout in order to cancel the application of the overshoot voltage, so that the voltage which is the product of the input voltage Vin and the output/input ratio has been lowered to a level below the voltage across the smoothing capacitor. Consequently, the charge discharged from the smoothing capacitor flows in reverse, thus creating a reverse current which flows from the output end to the input end of the DC-DC converter.
Such a reverse current is tremendously large even when the overshoot voltage is so small as to slightly exceed the normal output voltage Vout and, therefore, causes various problems.
The generation of such a large reverse current is attributable to the regulation characteristics as shown in FIG. 7A exhibited by a DC-DC converter having a conventional synchronous rectifier. To be more specific, a DC-DC converter of the type which produces an output voltage Vout through rectification/smoothing performed by a synchronous rectifier has such a regulation characteristic that the incremental variation (gradation) of the output voltage Vout to a decremental variation of the output current is not steep, as will be seen from FIG. 7A. Referring to FIG. 7A, the output current flows from the input end to the output end of the DC-DC converter in the region where the output current is positive (+), whereas the region where the output current is negative (xe2x88x92) is a reverse current region in which the current flows reverse from the output end to the input end of the DC-DC converter.
Due to the regulation characteristic shown in FIG. 7A, even a slight overshoot voltage Vx at the output end of the DC-DC converter causes a large reverse current Ix flowing reverse through the DC-DC converter.
Such a large reverse current causes a large electrical current stress and, hence, poses a risk of breakdown of a component of the DC-DC converter. A DC-DC converter of the type described also has a transformer and a choke coil for rectification and smoothing, in addition to the synchronous rectifier. The reverse current flowing through the on-period of the switching element produces large electromagnetic energy that accumulates in the choke coil and the transformer. Upon turning off of the switching element, the accumulated energy produces a high voltage that is applied to the switching element and the rectifying/smoothing synchronous rectifier, thereby posing a risk of destruction of the switching element and the synchronous rectifier. Thus, the large reverse current involves a problem of breakdown of the components of the DC-DC converter.
A plurality of DC-DC converters may operate in a parallel running manner in which a parallel connection of the DC-DC converters is connected to a load. In such parallel running of DC-DC converters, it is experienced that the DC-DC converters connected in parallel produce different levels of output voltages Vout. Such variation in the output voltage causes a reverse current flowing from a DC-DC converter producing higher level of the output voltage Vout to the DC-DC converter producing a lower level of output voltage Vout.
By way of example, it is assumed here that a DC-DC converter A producing the higher output voltage Vout has a regulation characteristic as shown by a solidline curve xe2x80x9cAxe2x80x9d in FIG. 7B, whereas a DC-DC converter xe2x80x9cBxe2x80x9d producing the lower output voltage Vout has a regulation characteristic as shown by a solid-line curve xe2x80x9cBxe2x80x9d in FIG. 7B. It is also assumed that the plurality of DC-DC converters taking part in the parallel running provide in cooperation a current IC supplied to the load. In this case, a reverse current Ib, attributable to the higher output voltage Vout produced by the DC-DC converter A is caused to flow in reverse in the DC-DC converter B that produces the lower output voltage Vout, with the result that a loss is caused in the DC-DC converter B due to the reverse current.
Meanwhile, the DC-DC converter A has to supply a current Ia (Ia=Ib+Ic) which is large enough to maintain the current Ic to be supplied to the load, by compensating for the insufficiency caused by the reverse flow of the current Ib. This leads to an increase in the electrical current flowing through the DC-DC converter A and, hence, the loss produced in this DC-DC converter. Thus, a difference in the output voltage Vout between different DC-DC converters that are running in parallel increases losses both in the DC-DC converter A producing the higher output voltage Vout and the DC-DC converter producing the lower output voltage Vout, thus causing a reduction in the efficiency of the circuit.
The present invention contemplates overcoming these problems. Accordingly, an object of the present invention is to provide a DC-DC converter having a synchronous rectifier which is improved to suppress any reverse current, thereby preventing breakdown of circuit components attributable to the reverse current, and to suppress increase of losses that may be caused by a reverse current occurring during parallel running of a plurality of DC-DC converters.
In order to achieve the above-described object, the present invention overcomes the foregoing problems by the following features. In accordance with the invention, there is provided a DC-DC converter having a synchronous rectifier which, in response to a switching operation of a switching element, performs a voltage conversion of an input voltage at an output/input conversion ratio that is determined by the switching operation of the switching element, and which delivers the converted voltage to a load, the DC-DC converter comprising: a reverse current detector detecting a reverse current which flows from the output to the input, and a reverse current suppressor which, when a reverse current is detected, controls the switching operation of the witching element so as to increase the output/input conversion ratio, thereby suppressing the reverse current.
Further, in accordance with the invention, there is provided a DC-DC converter wherein the switching element is so arranged that its switching operation is controlled based on a control voltage applied to control terminals of the switching element, and wherein the reverse current detector includes a control voltage detector detecting the control voltage of the switching element, inter-terminal voltage detecting means for detecting a voltage across the switching element as measured between a current input terminal and a current output terminal of the switching element, and a reverse current detection signal providing a reverse current detection signal when the control voltage is below a threshold voltage for turning the switching element on while the inter-terminal voltage across the switching element is at a low level as caused by a reverse current.
Further, in accordance with the invention, in the DC-DC converter, the reverse current detector detects the input current or the current flowing through the switching element, and detects that a reverse current is flowing on condition that the detected voltage is not higher than a predetermined set voltage.
Also, in accordance with the invention, the reverse current detector detects a current which flows in a current path and the direction of which is reversed when a reverse current exists, and determines that a reverse current exists based on the direction of the current in the current path.
In the DC-DC converter of the present invention having the features set forth above, when the reverse current detector has detected a reverse current, the reverse current suppressor controls the switching operation of the switching element so as to increase the output/input conversion ratio. This control performed by the reverse current suppressor provides the DC-DC converter of the present invention with a regulation characteristic which shows a steep incremental gradient of the output voltage with respect to an incremental variation of a reverse current. Therefore, when an overshoot voltage is imposed on the output end of the DC-DC converter, the reverse current attributable to the overshoot voltage is remarkably suppressed as compared with that experienced in the conventional arrangement.
This serves to substantially completely eliminate breakdown of parts attributable to a reverse current. In addition, when there is a variation in the output voltage among a plurality of DC-DC converters running in parallel, any reverse current attributable to such an output voltage variation is suppressed to reduce any loss incurred in the DC-DC voltage flowing through the DC-DC converter, thus suppressing reduction in the efficiency of the circuit.
In accordance with the present invention, the DC-DC converter has a reverse current detector and a reverse current suppressor which, when a reverse current is detected, operate to control the operation of a switching element in such a direction as to increase the output/input conversion ratio. This arrangement imparts to the DC-DC converter a peculiar characteristic which serves to suppress a reverse current when such a reverse current exists.
With the features set forth above, it is possible to provide a DC-DC converter having a synchronous rectifier which implements a revolutionary unique feature of suppressing reverse current caused by any overshoot voltage appearing at the output side of the DC-DC converter that has never been achieved heretofore.
As will be understood from the foregoing description, the DC-DC converter of the present invention can suppress any reverse current, thus eliminating almost completely various problems such as breakdown of circuit components which otherwise would be caused by a large reverse current, thus enhancing the reliability of the DC-DC converter over a long period of use.
When a plurality of DC-DC converters are used in a parallel-running fashion, a reverse current which may be caused by a variation in the output voltage among these DC-DC converters is suppressed by the aforesaid peculiar characteristic offered by the invention, thus avoiding increase in the loss which otherwise may be caused by a reverse current, thus preventing reduction in the efficiency of the entire circuit.
The reverse current suppressor may comprise a control voltage detector an inter-terminal voltage detector and a reverse current detection signal output device. The reverse current detector may be arranged to detect an input current or a current flowing through the switching element and to determine that a reverse current exists when the detected current is not greater than a predetermined set value. The reverse current detector also may be implemented by an element capable of detecting a current in a path of a current which reverses its direction when a reverse current exists, so that occurrence of a reverse current is determined based on the direction of the current detected in this path. All these forms of the invention enable detection of a reverse current by simple circuit arrangements, while achieving advantageous effects equivalent to those discussed above.
For the purpose of illustrating the invention, there is shown in the drawings several forms which are presently preferred, it being understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown.
Other features and advantages of the present invention will become apparent from the following description of the invention which refers to the accompanying drawings.